I use a full length BA pad. It would get a little tricky with some of the other pads out there but then again with a couple of protos im sure you could work out some way to cinch it down to be mass produced. I am more throwing this out there to see what someone or a few can come up with. I think I would be good at copying someone elses design. I am new to myog and while I can visualize the concept I have no idea how to sew it together.

A possible idea would to use a custom variable girth sleeping quilt, something where you would be able to remove the straps to attach to the bivy chair pad for the supporting straps, or some sort of removable compression system on a backpack; this would eliminate any weight penalty.

I guess an inflatable pad would give best support but you would be carrying extra weight.

the idea of using compression straps with a bivy to make a backpack... I think this a great idea (patting myself firmly on the back here) because the pack would be as big or as small as you desired and it would eliminate some weight.

I'm definitely looking for feedback on whether anyone has ever attempted this before or suggestions on how to complete, my idea was inserting the sleep mat as a back pad folding into pack size, using compression straps maybe sewing on some shoulder straps... dunno. Open idea.

Take a look at the Thermarest chair. I was given one as a gift, but have never used it. It is pretty clever. However, they do use a couple of poles inside the fabric to give it support on the sides. That maybe an important factor in making something workable.